1. technology and computing
2. consumer electronics
3. telephones
4. mobile phones

Wireless Data Communication Networks
Lecture 19: Introduction to GSM
Florida Institute of Technology
ECE Department
Outline
Brief history
GSM network architecture
Elements of the air interface organization
Florida Institute of Technology
ECE Department
Page 2
Global System for Mobile Communication
 Most popular cellular standard
 Started as Groupe Special Mobile
 2nd generation global digital standard
 Important time stamps:
o 1982 CEPT created Groupe Special Mobile
o 1987 MoU – 13 European countries
Official GSM logo
o 1989 Transferred to ETSI
o 1991 Publishing of Phase 1 specs
o 1991 Radiolinia Finland – first GSM network
o 1993 Telstra Australia – first non-European GSM network
o 1995 US implements GSM in PCS band
o 1996 Russia and China implement GSM
o Since 1996 tremendous growth all over the world
Florida Institute of Technology
ECE Department
Page 3
GSM in the World
GSM currently holds
about 80% of the market
share worldwide
Florida Institute of Technology
ECE Department
Page 4
GSM Network Layout
MSC
Area
BSS
BSS
BTS
Gateway
MSC
NSS
BSC
TRAU
PSTN
MSC
VLR
BTS
BTS
BSS
HLR
MSC
Area
MSC Area
PLMN - Public Land Mobile Network
GSM architecture is
first standardized
cellular system
architecture
 GSM network can be divided into three main subsystems:
 Base Station Subsystem - BSS
 Mobile Station Subsystem - MSS
 Network Switching Subsystem - NSS
Florida Institute of Technology
ECE Department
Page 5
GSM Network Components
VLR
BTS
D
B
BSC
HLR
C
MSC
BTS
AUC
BSC
EIR
E
BTS
F
MSC
VLR
BTS
PSTN
Um
Interface
B,C,D,E,F - MAP Interfaces
Florida Institute of Technology
A-Bis
Interface
A
Interface
BTS
BSC
MSC
VLR
HLR
AUC
EIR
-
 Network has many
functional components
 Components are integrated
through a network protocol
– MAP
 Majority of GSM interfaces
are standardized – allows
interworking of equipment
from different vendors
 Major equipment vendors:
Ericsson, Nokia-Siemens,
Alcatel-Lucent, Huawei,
Motorola, etc.
Base Station
Base Station Contoller
Mobile Sw itching Center
Visitor Location Register
Home Location Register
Authentification Center
Equipment Identity Register
ECE Department
Page 6
Overview of GSM - Mobile Station
 Offered as a phone for voice
services
 Data services will bring new
devices to the market
CONTROL
SECTION
ANTENNA
ASSEMBLY
TRANSCEIVER UNIT
Receive Audio
Signal
Processing
Display
Channel
Encoding
Interleaving
Message
Generator
Ciphering
RF
Processing
Antenna
RECEIVER
Duplexer
 Two functional parts:
 HW/SW radio
interface
 SIM
SIM
Control
Keyboard
Transmit Audio
Signal
Processing
Channel
Decoding
Deinterleaving
Message
Regenerator
Ciphering
RF
Processing
TRANSMITTER
GSM Mobile Architectural Diagram
• Two types of SIM (Subscriber Identity Module)
– Smart Card
– Plug-in
Florida Institute of Technology
ECE Department
Page 7
Overview of GSM - Base Transceiver Station
 BTS is a set of transceivers (TX/RX).
 GSM BTS can host up to 16 TX/RX.
 In GSM one TX/RX is shared by 8 users.
 The main role of TX/RX is to provide conversion between
traffic data on the network side and RF communication on
the MS side.
 Depending on the application, it can be configured as
megacell, macrocell, microcell, pico and femto cell, omni,
sectored, etc.
BTS
Typical BTS
installations
Florida Institute of Technology
ECE Department
Page 8
Overview of GSM - Base Station Controller
 Provides a small digital exchange
with some mobility tasks
 Connects to one or several BTS on
the Abis Interface
Database
Abis
Interface
Towards
BTS
 Connects to the MSC on the A
Interface
 Designed to offload most of the
radio link related processes from
the MSC
 Provides clock distribution to BTS
 Communicates with the OMC
Florida Institute of Technology
Switch
Matrix
A
Interface
Towards
MSC
Central Module
OMC
OMC - Operation and Maintenance
Center
In GSM, BSC handles majority RRM functions
• Handoff
• Power control
• Channel assignment
ECE Department
Page 9
Overview of GSM - TRAU
 TRAU is responsible for trans-coding
the user data from 16Kb/sec to
standard ISDN rates of 64Kb/sec.
 It can physically reside on either
BSC side or MSC side.
• If it resides on the MSC side, it provides
substantial changes in the backhaul – 4
users over a single T-1/E-1 TDMA
channel.
• TRAU, BSC and BTSs form Base
Station Subsystem (BSS)
Location of TRAU in GSM
Florida Institute of Technology
ECE Department
Page 10
Overview of GSM - MSC
• One of the MSC has an added
functionality for communication
with public network – Gateway
MSC (GMSC)
 Responsible for connecting the
mobile to the landline side
 GSM MSC is commonly designed
as a regular ISDN switch with
some added functionality for
mobility support
• All calls from the “outside
networks” are routed through
GMSC
 GSM Network can have more than
one MSC
BSC
MSC
GMSC
BTS
GSM MSC and Gateway MSC
Florida Institute of Technology
ECE Department
Page 11
Introduction to GPRS
Overview of GSM - HLR/AuC
 Database for permanent or semi-permanent data associated with the user
 Logically, there is only one HLR per network
 Typical information stored in HLR: International Mobile Service Identification
Number (IMSI), service subscription information, supplementary services,
current location of the subscriber, etc.
 HLR is usually implemented as an integral part of MSC
 AUC is an integral part of HLR responsible for ciphering and encryption.
 GSM specifies elaborate encryption schemes.
 There are three levels of the encryption:

A5/1 – Used by countries in Europe and USA

A5/2 – Used by countries and the so called COCOM list

No encryption – used by all other countries
Florida Institute of Technology
ECE Department
Page 12
GSM frequency bands
 14 bands defined
 Most dominant
o GSM-900 (P-GSM+ E-GSM)
o DCS-1800
o GSM-850 (US)
o PCS-1900 (US)
 It is common that a phone is
capable of multiband operation
o Dual band - 2 bands
o Quad band - 4 bands
Florida Institute of Technology
ECE Department
Page 13
GSM – Frequency <-> ARFCN calculations
ARFCN – Absolute Radio
Frequency Channel Number
2
2
Example – US PCS allocation
Florida Institute of Technology
ECE Department
Page 14
Overview of GSM - Air Interface
USER 1,
ARFCN1
 GSM is a FDMA/TDMA based
technology
USER 2,
ARFCN1
 Transmissions are discontinuous
USER 1
 Each user is assigned a timeslot
....
USER 8
ARFCN1
USER 8,
ARFCN1
 Each frequency is divided into eight
timeslots
USER 2
USER 6
USER 7
USER 8
USER 1
ARFCN2
 Each channel has a 200 kHz bandwidth
USER 9,
ARFCN2
USER 10,
ARFCN2
 Overhead signaling is required for
coordination and control
USER 16,
ARFCN2
 Information is sent in bursts
BTS
 Several types of bursts
GSM as a FDMA/TDMA Interface
Florida Institute of Technology
ECE Department
Page 15
Overview of GSM - Burst Types
3
Tail
57
Traffic/Signaling
1
26
1
57
3
Flag
Training Sequence
Flag
Traffic/Signaling
Tail
Normal Burst
 Used to carry information on both control and traffic channels
 Mixture of data and overhead
 GSM defines 8 training sequences assigned in color code mode
 Both on the forward and reverse link
3
39
64
39
3
Tail
Synchronization
Training Sequence
Synchronization
Tail
Synchronization Burst
 Facilitates the synchronization of the MS to the network at the base band
 Commonly referred to as S-burst
 Only on the forward link
 The same sync sequence is used in all GSM networks
Florida Institute of Technology
ECE Department
Page 16
Overview of GSM - Burst Types
8
41
Tail
Synchronization
36
3
Access Bits
Tail
Access Burst
 Used when the MS is accessing the system
 Shorter in length – burst collision avoidance
• GSM mobiles use slotted
ALOHA to access the system
• In the case of collision – a
hashing algorithm is provided
 Extended synchronization sequence
 Used only on the reverse link
3
142
3
Tail
Predefined Bit Sequence
Tail
Dummy Burst
• Supports MAHO
• Used to ensure constant power level of the broadcast control channel
• Only on the forward link
Florida Institute of Technology
ECE Department
Page 17
Overview of GSM - Burst Types
 Sometimes referred to as the F-burst
Power Spectrum Density
 Provides mobile with precise reference
to the frequency of the broadcast
control channel
 Inserting the F-bursts on the control
channel produces spectral peak 67.7
KHz above the central frequency of the
carrier
 Only on the forward link
BW = 200KHz
fc
fc+67.7 KHz
frequency
• Spectral characteristics of the
control channel.
• The peak in the spectrum allows
for easier MS network acquisition
Frequency synchronization burst
3
142
3
Tail
Fixed Bit Sequence (All zeros)
Tail
Florida Institute of Technology
ECE Department
Page 18
Overview of GSM - Physical Channels
 A GSM physical channel can carry several different types of logical channels
 Can be divided into two categories: traffic and signaling
 Signaling channels can be further categorized as:

Broadcast

Common Control

Dedicated Control
Group
Name
Function
BCH
BCCH
FCCH
SCH
PCH
AGCH
RACH
SDCCH
SAACH
FACCH
TCH
Broadcast control
Frequency correction
Synchronization
Paging
Access Grant
Random Access
Standalone dedicated control
Slow-associated control
Fast-associated control
Traffic data and voice
CCCH
DCCH
TCH
Florida Institute of Technology
ECE Department
Fwd.
Link
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Rev.
Link
No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
Page 19
Overview of GSM - Frame Hierarchy
 Different organization on the superframe level for different logical channels
Hyperframe
3 h 28 min 53 s 760 ms
0
1
2
3
4
5
6
7
2043
2044
2045
2046
2047
48
49
50
51 x 26 Superframe or 26 x 51 Superframe
6s 120 ms
0
1
2
3
4
5
6
7
46
47
48
49
50
0
1
2
3
4
5
6
7
21
22
23
24
25
26 Multiframe
120 ms
0
1
2
3
4
51 Multiframe
235.4 ms
23
24
25
0
1
2
0
3
4
5
1
6
2
3
4
7
1 TDMA Frame
4.615 ms
Florida Institute of Technology
ECE Department
Page 20
GSM Time Division Duplex
 Communication on the forward and reverse link does not happen
simultaneously
 Delay of three slots between TX and RX
 Time division duplexing avoids RF duplexer at the RF stage
o Reduces the cost of mobile
o Saves battery
Forward Link - BTS Transmits
0
1
2
3
4
5
6
7
0
3
4
5
Reverse Link - MS Transmits
5
Florida Institute of Technology
6
7
0
1
2
ECE Department
Page 21
Review questions
 What are the elements of GSM
system?
 What is a BTS?
 What is a BSC?
 What is a MSC?
 What are four most prevalent
frequency bands for GSM
deployment?
 Explain GSM FDMA/TDMA
access?
 What is a HLR?
 How many time slots are on one
GSM ARFCN?
 What is a VLR?
 What is an ARFCN?
 What is TRAU?
 Name 2 basic channel types in
GSM?
 What is a SIM card?
 What is the Abis?
Florida Institute of Technology
 Name five burst types used in
GSM?
ECE Department
Page 22
Review questions
 Draw GSM TDMA hierarchy for
traffic channel.
 Draw GSM TDMA hierarchy for
control channel.
 What is the bandwidth of a GSM
channel?
 What is the difference between
DCS-1800 and PCS-1800?
 What is a quad-band phone?
 Do GSM mobiles use RF
duplexers? Please explain.
Florida Institute of Technology
ECE Department
Page 23